This invention relates, in general, to semiconductor devices, and more particularly, to a method of making a bipolar transistor with improved low current gain.
A well known problem that exists in bipolar transistors is the degradation of the gain at low currents. Low current gain degradation is exhibited when the emitter-base P-N junction is under reverse bias breakdown testing, or ageing. The reverse bias emitter-base P-N junction ageing induced reduction of low current gain is due to minority carrier base current lost by an increase in surface recombination. This degradation can also occur when a carrier recombination current is generated in the emitter-base junction transition region in the bulk. Although this degradation of the gain at low currents is not detrimental to the operation of the device for many applications, it is considered to be an indication of poor reliability and instability. A degradation of the gain is also exhibited at high currents when a surface channel is formed parallel to the emitter-base region.
An apparent reduction in the amount of degradation of the gain at low currents has been achieved by the use of shunts, both external and internal to the semiconductor device. These shunts, however, do not affect the high current gain degradation. An internal shunt is commonly an emitter-base resistive shunt deposited on a thermal oxide, which overlays the emitter-base junction, and is connected to emitter and base metal. High value thin film resistors, such as amorphous silicon, have been used as the resistive shunt. One of the problems that arises is that considerable additional processing steps are necessary to incorporate such resistive shunts on the silicon dioxide layer of some integrated circuit structures. In addition, if amorphous silicon is used, subsequent processing temperatures must be kept low in order to avoid the crystallization of the amorphous silicon.
By now it should be appreciated that it would be advantageous to provide a bipolar transistor which exhibits no degradation of low current gain and is simple to manufacture.
Accordingly, it is an object of the present invention to provide an improved bipolar transistor exhibiting no degradation of the low current gain under reverse bias breakdown testing.
Another object of the present invention is to provide a bipolar transistor wherein the formation of a surface channel is prevented.
A further object of the present invention is to provide a bipolar transistor having stable emitter-base characteristics fabricated with minimal additional processing steps.
Yet another object of the present invention is to provide a bipolar transistor with a shunt which prevents reverse bias emitter-base P-N junction ageing induced reduction of low current gain.